1. Field of the Invention
The present invention relates to an electron microscope for imaging a specimen by focusing an electron beam (hereinafter may be referred to as the “electron probe” or simply as the “probe”) onto the specimen, scanning the probe over the specimen, detecting the electrons transmitted through the specimen by an electron detector, and visualizing the output signal from the detector in synchronism with the electron beam scanning. More particularly, the invention relates to a method of measuring aberrations by an electron microscope equipped with an illumination system aberration corrector by the use of a Ronchigram and to a method and apparatus for correcting aberrations.
2. Description of Related Art
In transmission electron microscopy, a method of imaging a specimen by focusing an electron beam onto the specimen, scanning the beam over the specimen, detecting the electrons transmitted through the specimen by an electron detector, and displaying the output signal from the detector as a visible image in synchronism with the electron beam scanning is known as STEM (scanning transmission electron microscopy) imaging. The spatial resolution of STEM images is affected by various aberrations in the electron beam hitting the specimen. In recent years, apparatus capable of obtaining smaller electron beam diameters than heretofore have been put into practical use by incorporating an aberration corrector into the illumination system, the corrector being capable of correcting spherical aberration. The following two methods are known to measure aberrations in electron beams in such apparatus.
1) Method of correcting aberrations using a probe profile calculated by Fourier analysis. An image of a just focus and an underfocused (or overfocused) image are taken from dark field images of a reference specimen of gold particulates on the order of nanometers. A probe profile is calculated from the image of the just focus and from the underfocused or overfocused image by Fourier analysis, and aberrations are estimated. Parameters of various deflection systems and a stigmator are varied from the estimated aberrations, thus correcting the aberrations. This method uses no Ronchigram. The Ronchigram is an image of a specimen projected to an infinitely distant point as viewed from the specimen (back focal plane) by means of an electron beam focused onto the specimen in the STEM imaging mode.
2) A Ronchigram of a reference specimen (particulates of gold) is created and observed. Aberrations are calculated from variations in magnification caused by positional shift across the Ronchigram (in a quite small angular region). When the variations in the magnification due to shifting are calculated, the electron beam is moved across the specimen. The amount of movement of the Ronchigram made between, before and after the movement of the beam is used. The parameters of the systems of deflection and a stigmator are varied using the calculated aberrations. In this way, the aberrations are corrected. This method uses a Ronchigram.
One known apparatus of this kind, for example, as described in U.S. Patent Application Pub. No. 2003/0001102 images an object by means of a beam of particles focused onto the object, recording the image, repeating the process steps carried out until the recording step using underfocused and overfocused beams, Fourier-transforming the images, dividing the Fourier transform of the overfocused image by the Fourier transform of a focused image, inverse transforming the quotient (result of the division), dividing the Fourier transform of the underfocused image by the Fourier transform of the focused image, inverse transforming the result of the division, determining a brightness profile of the probe (i.e., images of the light sources of overfocused and underfocused images), determining the asymmetry of the contour about the center of the image, the width of the contour (especially, the half value width), and/or the curvature of the contour about the center, and using the differences in the probe contour for the different parameters to determine the aberrations in the image.
Another known apparatus using a beam of charged particles, for example, as described in U.S. Pat. No. 6,552,340 is designed to minimize the optical aberrations and includes a source of the charged particles, a probe-forming system of charged-particle lenses, a plurality of two-dimensional detectors, a power supply, a computer, and preferred software. This apparatus automatically corrects aberrations.
The above-described known methods have the following problems.
Any method of the above-described techniques uses a reference specimen. Where an actual specimen is observed using this method, it is necessary to replace the specimen. Furthermore, in order to search for a desired specimen location to be observed, the operating mode may be switched from STEM mode to TEM mode. This induces drifts of varying extents in the systems of deflection and stigmator.
When a specimen is observed in practice, various aberrations which should have been corrected vary due to drift (i.e., timewise variations of the magnetic field produced by the lenses). There is the problem that ultrahigh-resolution images cannot be obtained due to the introduced aberrations.